Electrochemical cells commonly contain a negative electrode (anode) and a positive electrode (cathode), an ion-permeable separator therebetween and an electrolyte in contact with both of the electrodes. Typical electrolytes can be aqueous-based or non-aqueous organic solvent-based liquid electrolytes or polymeric electrolytes. There are two basic types of electrochemical cells, a primary and a secondary (rechargeable) electrochemical cell. A primary electrochemical cell is discharged to exhaustion only once. A secondary electrochemical cell, however, is rechargeable and thus can be discharged and recharged multiple times.
Primary lithium electrochemical cells typically employ an anode of lithium metal or lithium alloy, preferably a lithium-aluminum alloy; a cathode containing an electrochemically active material consisting of a transition metal oxide or chalcogenide, preferably manganese dioxide; and an electrolyte containing a chemically stable lithium salt dissolved in an organic solvent or a mixture of organic solvents.
The lithium anode is preferably formed from a sheet or foil of lithium metal or lithium alloy without any substrate. A lithium primary cell referenced hereinafter as having an anode comprising lithium shall be understood to mean an anode of lithium metal or a lithium alloy. If a lithium-aluminum alloy is employed, the aluminum is present in a very small amount, typically less than about 1 wt % of the alloy. The addition of aluminum primarily serves to improve the low temperature performance of the lithium anode in lithium primary cells.
Manganese dioxides suitable for use in lithium primary cells include both chemically produced manganese dioxide known as "chemical manganese dioxide" or "CMD" and electrochemically produced manganese dioxide known as "electrolytic manganese dioxide" or "EMD". CMD can be produced economically and in high purity, for example, by the methods described by Welsh et al. in U.S. Pat. No. 2,956,860. However, CMD typically does not exhibit energy or power densities in lithium cells comparable to those of EMD. Typically, EMD is manufactured commercially by the direct electrolysis of a bath containing manganese sulfate dissolved in a sulfuric acid solution. Processes for the manufacture of EMD and representative properties are described in "Batteries", edited by Karl V. Kordesch, Marcel Dekker, Inc., New York, Vol. 1, 1974, pp.433-488. Manganese dioxide produced by electrodeposition typically is a high purity, high density, "gamma(.gamma.)-MnO.sub.2 " phase, which has a complex crystal structure containing irregular intergrowths of a "ramsdellite"-type MnO.sub.2 phase and a smaller portion of a beta(.beta.)- or "pyrolusite"-type MnO.sub.2 phase as described by dewolfe (Acta Crystallographica, 12, 1959, pp.341-345). The gamma(.gamma.)-MnO.sub.2 structure is discussed in more detail by Burns and Burns (e.g., in "Structural Relationships Between the Manganese (IV) Oxides", Manganese Dioxide Symposium, 1, The Electrochemical Society, Cleveland, 1975, pp. 306-327), which is incorporated herein by reference.
The structural disorder present in the crystal lattice of gamma(.gamma.)-MnO.sub.2 includes non-coherent lattice defects, such as stacking faults, micro-twinning, Mn.sup.+4 cation vacancies, Mn.sup.+3 cations from reduction of Mn.sup.+4 cations, lattice distortion introduced by the Mn.sup.+3 cations (i.e., Jahn-Teller effect), as well as compositional non-stoichiometry as described, for example, by Chabre and Pannetier (Prog. Solid State Chem., Vol. 23, 1995, pp. 1-130) and also by Ruetschi and Giovanoli (J. Electrochem. Soc., 135(11), 1988, pp. 2663-9), both incorporated herein by reference.
Ruetschi has proposed a chemical formula for .gamma.-MnO.sub.2 which is based on a structural defect model (J. Electrochem. Soc., 131(12), 1984, pp. 2737-2744). In this model, the crystal lattice structure of .gamma.-MnO.sub.2 can be described as comprising an anion sublattice consisting of a close-packed array of oxygen anions and a corresponding cation sublattice consisting of an array of predominantly Mn.sup.+4 cations, some Mn.sup.+3 cations, and occasional Mn.sup.+4 cation vacancies. Further, in order to maintain overall electroneutrality of the .gamma.-MnO.sub.2 crystal lattice, the positive charge deficiencies resulting from the presence of Mn.sup.+3 cations as well as the Mn.sup.+4 cation vacancies must be compensated. This can be accomplished by substitution of OH.sup.- (hydroxyl) ions for O.sup.-2 ions in the anion sublattice, which is nominally equivalent to protonation of O.sup.-2 anions by hydrogen ions. Thus, for each Mn.sup.+3 cation present, one hydrogen ion must be introduced into the lattice to maintain charge compensation. Similarly, for each Mn.sup.+4 cation vacancy, four hydrogen ions must be introduced to maintain the overall electroneutrality. The OH.sup.- anions formed are also referred to as "structural" or "lattice water". Thus, the chemical formula for .gamma.-MnO.sub.2 can be represented as: EQU Mn.sup.+4.sub.1-x-y Mn.sup.+3.sub.y {character pullout}.sub.x O.sub.2-4x-y (OH).sub.4x+y (1)
wherein {character pullout} stands for Mn.sup.+4 vacancies; x is the fraction of Mn.sup.+4 vacancies; and y is the fraction of Mn.sup.+3 cations. Also, Reutschi has proposed that hydrogen ions associated with the Mn.sup.+3 cations are mobile while hydrogen ions associated with the immobile Mn.sup.+4 cation vacancies are localized.
It is theorized by the Applicants herein of the present Patent Application that such mobile hydrogen ions present in the .gamma.-MnO.sub.2 lattice can be advantageously substituted by lithium cations by way of an ion-exchange process prior to the traditional heat-treatment without further reduction of Mn.sup.+4 to Mn.sup.+3, in contrast to typical reductive lithium insertion processes of prior art. Although Reutschi has proposed that such hydrogen ions are mobile, neither a particular process for ion-exchanging the mobile hydrogen ions by lithium cations nor the desirability of such an ion-exchange process was disclosed.
Ruetschi further theorized that the number of mobile hydrogen ions depends on both the degree of oxidation of the manganese atoms and total lattice water content and can be determined experimentally. For example, it is theorized by the Applicants herein of the present Patent Application that according to Equation (1) hereinabove, about 20% of the lattice hydrogen ions of an EMD having a nominal chemical formula of MnO.sub.1.96.0.23 H.sub.2 O, for example, can be ion-exchanged by lithium cations as shown in Equation (2): EQU Li.sup.+ +Mn.sup.+4.sub.0.84 Mn.sup.+3.sub.0.73 {character pullout}.sub.0.087 O.sub.1.579 (OH).sub.0.421 {character pullout}Li.sub.0.08 MnO.sub.2.0.18 H.sub.2 O (2)
Electrochemical manganese dioxide (EMD) is the preferred manganese dioxide for use in primary lithium cells. However, before it can be used, it must be heat-treated to remove residual water. The term "residual water", as used herein includes surface-adsorbed water, noncrystalline water (i.e., water physisorbed or occluded in pores), as well as lattice water. Heat-treatment of EMD prior to its use in lithium cells is well known and has been described by Ikeda et al. (e.g., in "Manganese Dioxide as Cathodes for Lithium Batteries", Manganese Dioxide Symposium, Vol. 1, The Electrochemical Society, Cleveland, 1975, pp. 384-401) and is incorporated herein by reference.
EMD suitable for use in primary lithium cells can be heat-treated at temperatures between about 200 and 350.degree. C. as taught by Ikeda et al. in U.S. Pat. No. 4,133,856. This reference also discloses that it is preferable to heat-treat the EMD in two steps. The first step is performed at temperatures up to about 250.degree. C. in order to drive off surface and non-crystalline water. The EMD is heated in a second step to a temperature between about 250 and 350.degree. C. to remove the lattice water. This two-step heat-treatment process improves the discharge performance of primary lithium cells, primarily because surface, non-crystalline, and lattice water are all removed. An undesirable consequence of this heat-treatment process is that EMD having the .gamma.-MnO.sub.2 -type structure, is gradually converted to EMD having a gamma/beta (.gamma./.beta.)-MnO.sub.2 -type structure. The term "gamma/beta-MnO.sub.2 " as used in the art reflects the fact (as described by Ikeda et al.) that a significant portion of the .gamma.-MnO.sub.2 (specifically, the ramsdellite-type MnO.sub.2 phase) is converted to .beta.-MnO.sub.2 phase during heat-treatment. At least about 30 percent by weight and typically between about 60 and 90 percent by weight of the ramsdellite-type MnO.sub.2 phase is converted to .beta.-MnO.sub.2 during conventional heat treatment of .gamma.-MnO.sub.2 as taught, for example, in U.S. Pat. No. 4,921,689. The resulting .gamma./.beta.-MnO.sub.2 phase is less electrochemically active than an EMD in which the .gamma.-MnO.sub.2 phase contains a higher fraction of ramsdellite-type MnO.sub.2 relative to .beta.-MnO.sub.2. Thackeray et al. have disclosed in U.S. Pat. No. 5,658,693 that cathodes containing such .beta.-MnO.sub.2 -enriched phases exhibit less capacity for lithium uptake during discharge in lithium cells.
One consequence of the electrodeposition process used to prepare EMD is that the formed EMD typically contains residual surface acidity from the sulfuric acid of the electrolytic bath. This residual surface acidity must be neutralized, for example, with basic aqueous solution, before the EMD can be used in cathodes for primary lithium cells. Suitable aqueous bases include: sodium hydroxide, ammonium hydroxide (i.e., aqueous ammonia), calcium hydroxide, magnesium hydroxide, potassium hydroxide, lithium hydroxide, and combinations thereof. Typically, commercial EMD is neutralized with a strong base such as sodium hydroxide because it is highly effective and economical.
An undesirable consequence of the acid neutralization process is that alkali metal cations can be introduced into ion-exchangeable sites on the surface of the EMD particles.
For example, when sodium hydroxide is used for acid neutralization, sodium cations can be trapped in the surface sites. This is especially undesirable for EMD used in cathodes of primary lithium cells because during cell discharge the sodium cations can be released into the electrolyte, deposit onto the lithium anode, and degrade the lithium passivating layer. Further, the deposited sodium cations can be reduced to sodium metal, react with the organic electrolyte solvents, and generate gas, thereby substantially decreasing the storage life of the cells.
A process for converting commercial grade EMD that has been neutralized with sodium hydroxide to the lithium neutralized form is disclosed by Capparella et al. in U.S. Pat. No. 5,698,176 and related Divisional U.S. Pat. No. 5,863,675. The disclosed process includes the steps of: (a) mixing sodium hydroxide neutralized EMD with an aqueous acid solution to exchange the sodium cations with hydrogen ions and produce an intermediate with reduced sodium content; (b) treating the intermediate with lithium hydroxide or another basic lithium salt to exchange the hydrogen ions with lithium cations; (c) heat-treating the lithium ion-exchanged EMD at a temperature of at least about 350.degree. C. to remove residual water. However, Capparella et al. disclose that "contacting particulate EMD with high pH lithium hydroxide solution may also serve to introduce lithium ions into the crystal lattice of the MnO.sub.2, thereby altering the crystal structure into a form which is not useful as a cathode active material". Further, Capparella et al. specifically teach against treatment of an aqueous suspension of EMD with lithium hydroxide to a final pH greater than 7.5 since such treatment was disclosed to destroy EMD particle integrity and produce sub-micron size MnO.sub.2 particles that were difficult to process.
A method for preparing a lithiated manganese dioxide and its use in electrochemical cells is disclosed by Dahn et al. in U.S. Pat. No. 4,959,282. The disclosed method involves the steps of: (a) forming a slurry of EMD in an aqueous solution of a lithium salt selected from LIOH, Li.sub.2 O, and LiNO.sub.3 at room temperature; (b) evaporating water from the stirred slurry at 100.degree. C. to obtain a dry intermediate having lithium salt deposited on particle surfaces as well as within pores; (c) heat-treating the dry intermediate at between 300 and 450.degree. C. for about 30 minutes to obtain a lithiated manganese dioxide having the formula Li.sub.y MnO.sub.2, wherein y is about 0.33 to 0.43. During heat-treatment, the .gamma.-MnO.sub.2 crystal structure was disclosed to convert to a new structure related to that of .gamma.-MnO.sub.2 having lithium ions intercalated in the crystal lattice that was referred to as "X-phase". However, the disclosed method produces a lithium manganese oxide having substantially higher lithium content than the lithiated manganese dioxide of the present invention.
Wang et al. disclosed a method for preparing lithium manganese oxide having a spinel-type structure in U.S. Pat. No. 5,753,202. As described therein, the spinel lithium manganese oxide is intended for use in lithium rechargeable cells, specifically lithium ion rechargeable cells. The disclosed method involves the steps of: (a) treating a manganese oxide (e.g., EMD) with a lithium salt (e.g., lithium hydroxide or nitrate) either in aqueous solution or in the solid state at a temperature between about 4 and 400.degree. C. to form an intermediate lithiated manganese oxide (e.g., Li.sub.x MnO.sub.2, 0.015&lt;x&lt;0.2); (b) heating the intermediate at between about 550 and 600.degree. C. to form a lithium manganese sesquioxide (viz., Li.sub.x Mn.sub.2 O.sub.3); (c) mixing the sesquioxide with additional lithium salt (e.g., Li.sub.2 CO.sub.3); (d) heating the mixture at between about 650 and 900.degree. C. to form a lithium manganese oxide having a spinel structure. The disclosed method is distinguishable over that of the present invention in that the method of the present invention does not produce any detectable amount of lithium manganese oxide having a spinel structure. Also, the lithiated manganese dioxide of the present invention is applied directly as a cathode active material in a primary lithium cell, in distinction to the intermediate lithiated manganese oxide of Wang et al. that must be converted to a lithium manganese oxide having a spinel structure before inclusion in the cathode of a lithium ion rechargeable cell. Furthermore, there was no contemplation of using the intermediate product of Wang et al. as a cathode active material in a primary lithium cell.
A method for preparing a manganese dioxide consisting essentially of ramsdellite-type MnO.sub.2 containing a minor portion of .beta.-MnO.sub.2 and the use thereof as an active cathode material is disclosed in U.S. Pat. No. 5,658,693.
The preferred method includes the steps of: (a) heating a stoichiometric lithium manganese oxide (e.g., LiMn.sub.2 O.sub.4) having a spinel structure in 2.6M aqueous sulfuric acid at 95.degree. C. for 2 days; (b) separating the intermediate product from the liquid; (c) drying the intermediate product overnight at 100.degree. C.; (d) heat-treating the intermediate product at a temperature below 400.degree. C. However, heat-treatment above about 300.degree. C. is disclosed to cause conversion of the ramsdellite-type MnO.sub.2 to .beta.-MnO.sub.2. It was further disclosed that heat-treatment at temperatures above 300.degree. C. but less than about 370.degree. C. in the presence of a lithium salt such as LIOH or LINO.sub.3 produces a lithium-stabilized ramsdellite-type MnO.sub.2 having the nominal composition Li.sub.2x MnO.sub.2+x, wherein 0.ltoreq.x.ltoreq.0.2, with only a minor portion of .beta.-MnO.sub.2 present. However, the disclosed method is overly complicated and inefficient to permit commercialization.
A method for preparing a lithiated manganese oxide has been disclosed in unexamined Japanese patent application JP62-160657, wherein a manganese dioxide was immersed in a highly alkaline aqueous solution containing .gtoreq.0.5M lithium ions at room temperature for 100 hours, collected, washed with water, and heat-treated at between 360.degree. C. and 430.degree. C. for 20 hours. Another related method for preparing a lithiated manganese oxide was disclosed in Japanese patent application JP52-073328, wherein EMD powder was immersed for about 24 hours in a saturated aqueous solution of lithium hydroxide, separated by filtration, and heat-treated at between 200 and 400.degree. C. for about four hours. A lithium button cell containing the lithiated EMD was disclosed to give a very flat discharge curve with less decrease in capacity after room temperature storage for one year than heat-treated EMD not immersed in lithium hydroxide solution.
Furukawa et al. in U.S. Pat. No. 4,758,484 claim a method for preparing a composite cathode material for lithium rechargeable cells wherein a mixture of manganese dioxide and a lithium salt selected from lithium hydroxide, lithium nitrate, lithium phosphate, lithium carbonate, and lithium oxide, having a Li/Mn mole ratio of from 0.11 to 2.33, is heat-treated at between 300 and 430.degree. C., preferably 350 and 430.degree. C. The product was disclosed to include a mixture of electrochemically non-active Li.sub.2 MnO.sub.3 and active lithiated manganese dioxide.
Treatment of a mixture of a manganese dioxide and lithium hydroxide or another lithium salt in a Li/Mn mole ratio of from 1:99 to 30:70 at a temperature between 170 and 280.degree. C. was disclosed in unexamined Japanese patent publication Hei 8-213018. A primary lithium cell including the treated product was disclosed to give discharge capacity greater than that for untreated manganese dioxide. In distinction with the present invention, the reference does not teach heat-treatment at temperatures greater than 280.degree. C. and, in fact, discourages heat-treatment at temperatures above 300.degree. C. However, related Japanese Patent Application JP08-115728 discloses sintering a mixture of manganese dioxide and a lithium salt selected from LiOH, Li.sub.2 CO.sub.3, LiNO.sub.3, and Li.sub.2 O at a temperature between 150 and 400.degree. C. The resulting "surface-improved" manganese dioxide containing about 1 to 15 mole % Li was included in the cathode of a primary lithium cell and disclosed to provide improved low temperature (-20.degree. C.) performance. The reference also teaches that manganese dioxide having lithium content &lt;1 mole % does not provide improved low temperature performance, whereas lithium content &gt;15 mole % decreases discharge capacity.
In yet another method for preparing active cathode materials for nonaqueous secondary cells claimed in unexamined Japanese Patent Application JP01-272051, MnO.sub.2 powder is heated with a lithium salt above the melting point of the lithium salt but below 430.degree. C. Suitable lithium salts are claimed to include LiNO.sub.3, LiClO.sub.4, LiBH.sub.4, and LiNH.sub.2. However, since LiBH.sub.4 and LiNH.sub.2 are well known to be very strong reducing agents and MnO.sub.2 is a very strong oxidizing agent, the claimed high temperature sintering is expected to result in a strongly exothermic reaction. Further, at the preferred Li/Mn mole ratios of from 0.11 to 2.33, the product is disclosed to contain a substantial amount of electrochemically non-active Li.sub.2 MnO.sub.3 as a second phase.
Thus, even though considerable effort has been expended, as evidenced by the cited prior art, the methods used to prepare active cathode materials including both lithium and manganese dioxide require additional refinement in order to substantially improve performance of primary electrochemical cells incorporating such active cathode materials.